1. Field of the Invention
The invention relates to electrohydraulic servovalve structures and methods. More specifically the invention refers to a high frequency response, second stage or pilot valve with electrical position feedback for use in a three stage, electrohydraulic servovalve. The feedback is developed by a non contacting, electromagnetic, proximity type sensor in the second stage pilot valve having a spool member. The sensor which is separated from the spool by a window is operable to convert spool position into an electrical signal. Sensor electronics for converting a high impedance signal to a lower impedance signal are provided on the pilot valve adjacent the sensor to increase the distance the signal representative of spool position may be effectively transmitted.
2. Description of the Prior Art
In seismic exploration work, a very high frequency response, rugged, high flow, electrohydraulic servovalve is needed. Electrohydraulic servovalves for such work are often three stage valves including a first stage force motor, a second stage, pilot valve and a third stage, metering valve.
Prior electrohydraulic servovalves used in seismic exploration requiring feedback from the pilot valve to the force motor electrical input have been limited in the pilot valve in that, in the past, developing an electrical signal representative of the position of the valve member of the pilot valve has placed some physical restraint on the movement of the valve member. Thus, for example, linear voltage differential transformers require a physical connection to the valve member whereby friction is created on side loading the linear voltage differential transformer.
Also the mass of the core of the linear voltage differential transformer attached to the valve member in the prior pilot valve structures increases the inertia of the valve members, thereby reducing the frequency response of the valve.
Further, the larger the volume of oil required to operate the pilot valve of such electrohydraulic servovalves, the lower the frequency response of the valve. With prior electrohydraulic servovalves, the pilot stage has had a greater than necessary oil volume therein as for example in pilot valves wherein linear voltage differential transformers are utilized to provide an electrical feed back signal.
In addition prior electrohydraulic servovalves have not had as high a frequency response as desired due to the frequency response of the force motor utilized in the first stage thereof. Thus, with force motors of the past, the armatures have not been as light as possible, the flexure tubes utilized therein have not been as stiff as desired, the inductance of the force motors has been higher than desired and dynamic balance of the armature with the drive arm has not normally been accomplished.
The above noted deficiencies in the prior art have resulted in three stage electrohydraulic servovalves which do not have as high a frequency response as desired for many applications, and in particular seismic exploration work.